This invention relates to optical junction devices for joining optical fibers, and it is primarily concerned with the dividing and/or combining of light propagating in optical fibers for effecting a distribution of light between a number of discrete paths or collection of light from multiple paths into a single path.
Optical waveguides, which are a promising transmission medium for optical communications systems, normally consist of an optical fiber having a core of transparent material having a refractive index n.sub.1 surrounded by a layer of transparent cladding material having a refractive index n.sub.2 which is lower than n.sub.1. Although the theory of optical waveguides has been known for some time, practical optical waveguides that do not absorb an excessive amount of transmitted light have been developed only recently. For example, U.S. Pat. No. 3,659,915 issued to R. D. Maurer et al. discloses a low loss optical waveguide comprising a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding. Other known optical waveguide structures include multiclad fibers and fibers having a gradient refractive index.
In optical communication systems using optical waveguides there are many occasions when it is useful to distribute the optical signal propagating in a single waveguide into two or more waveguides. Conversely, there are occasions when optical signals propagating in two or more waveguides need to be coupled into a single waveguide. In accordance with a further extension of this concept, the optical signals propagating in two or more waveguides can be mixed, the composite signal being coupled into two or more additional waveguides.
Optical junction devices are known wherein at least one optical fiber is coupled to a plurality of optical fibers, a few of such devices having been disclosed in U.S. Pat. Nos. 3,455,625 issued to C. H. Brumley et al., 3,453,036 issued to C. H. Swope et al., 3,870,396 issued to J. G. Racki et al., 3,870,398 issued to R. E. Love, and 3,901,581 issued to F. L. Thiel. In junction devices such as these light is coupled through a transparent medium, thus introducing Fresnel losses at the fiber-medium interfaces.